Hydrogels are polymers that can absorb and retain water, many times their own mass. So called “smart” polymers respond to external stimuli like temperature, pH, etc, with a change in their size or shape. Thermoreversible hydrogels are a subclass of hydrogels that can reversibly swell or shrink with a change in temperature.
Hydrogels have been shown to be highly biocompatible, due to their ability to sorb large amounts of water. They are usually available in the form of dense, porous materials. Normally, hydrogels are molecularly crosslinked through covalent or ionic bonds. They are also classified in terms of their solubility or swelling characteristics. In particular, the thermoreversible hydrogel exhibits decreased (increased) solubility or swelling in water as the temperature is increased (decreased), due to reversible phase transformation at the lower critical solution temperature. Hydrogels are “smart” materials because they can respond with a change of shape or size to an external stimulus of a wide variety of parameters, such as temperature, pH, moisture, magnetic or electric field, or even a pulse of an intense laser beam. Conventional methods of synthesis and fabrication of these hydrogels results in gel-like products, the shape or size of which cannot be controlled as desired. Conventional methods of synthesis and fabrication of these hydrogels is carried out by milling down bulk hydrogels, or by synthesizing them in containers of the desired shape or size.
Free radical precipitation polymerization, which occurs above the lower critical solution temperature is characterized by: (1) a poor solvent environment to aid in polymer precipitation; (2) the polymerization taking place above the lower critical solution temperature, wherein the phase envelope deepens with an increase in temperature and molecular weight of the polymer; and (3) an exothermic chain reaction drives the polymer deep into the phase envelope, with the reaction being predominantly diffusion controlled. Conventional polymerizing systems occur below the single-phase miscible region. Deep X-ray lithography is conventionally used to produce metallic microstructures based on LIGA (a German acronym for Lithographic Galvanoformung Abformung, or lithography electroplating molding). Major aspects of deep x-ray lithography include: (1) mask making, i.e. photolithography and electroplating; (2) spin coating of the resist (polymer) layer on the substrate (for example Poly(methyl methacrylate or SU-8 (3) exposure through the x-ray mask; and (4) developing the resist layer.
There is therefore a need for a method to produce “smart” polymer patterns with unique responsive properties. A nanoscale technique is needed for polymer synthesis that is cost-effective and more efficient then the current macro or micro-scaled techniques.